Ultra high vacuum device



June 16, 1964 .1. T. MARK ULTRA HIGH VACUUM DEVICE 2 Sheets-Sheet 1 Filed Oct. 2, 1959 INVENTOR.

JOHN T. MARK ATTORNEY June 16, 1964 J. T. MARK 3,137,551

ULTRA HIGH VACUUM DEVICE I Filed Oct. 2, 1959 2 Sheets-Sheet 2 INVENTOR.

JOHN T. MARK B ATTORN EY United States Patent 3,137,551 ULTRA. HiGi-l VACUUM DEVTCE .iohn T. Mark, Lancaster, Pa, assignor, by mesne assignments, to the United States'ct America as represented by the United States Atomic Energy Commission Filed Get. 2, 1959, Ser. No. 3438l3 6 Claims. (Cl. 555-269) This invention relates to a system for producing an ultra high vacuum and particularly to an improved liquid nitrogen trap for use in an ultra high vacuum system.

In order to design a vacuum system which is capable of efficiently evacuating a large space, and one which is capable of attaining an ultra high vacuum, it has been found that the trap, or traps, used within the system: (1) should offer a minimum impedance in the pumping system; (2) all of the walls of the trap should remain constantly cold; (3) there should be no path around the cold surfaces whereby molecules of a diffusion pump fluid could migrate from a low vacuum region to a high vacuum region; and (4) the consumption of the cooling fluid in the trap should be held to a minimum.

It is therefore an object of this invention to provide a new and improved trap for an ultra high vacuum system.

It is a further object of this invention toprovide a novel ultra high vacuum system including an improved trap that is capable of maintaining a vacuum of the order of 2 l0 mm. of Hg.

These and other objects have been accomplished in accordance with thisinvention by providing an ultra high vacuum system including a novel liquid nitrogen trap. The novel trap is formed by suspending a tank, which is made of a high thermal capacity metal, inside of a vacuum chamber from the lower edge of a thin walled tube of low heat conductivity within which the tank is carried. The tank, in which a coolant fluid may be circulated, preferably includes a plurality of cooled interrupted short chevron baffles through which the evacuated atmosphere is drawn. The bafiles are also made of a high thermal capacity metal.

The invention will be more clearly understood by reference to the accompanying two sheets of drawings where- FIG. 1 is a partially schematic view of an ultra high vacuum system utilizing this invention; and,

FIG. 2 is a sectional view of a novel vacuum trap in the arrangement of FIG. 1.

Referring specifically to FIG. 1, the vacuum system shown comprises a mechanical pump 1d which may be a pump of a type commercially available from the Kinney Mwufacturing Company known as the Kinney KC 46 two stage, gas ballast, mechanical pump. Connected to the mechanical pump Iii through a valve 11 is a mercury dififusion type booster pump 12 which may be of the type that is commercially available from the Consolidated Electrodynamics Corporation known as the MEG 900 mercury booster pump.

Connected to the booster pump 12 is a mercury diffusion pump 14 which may be of the type commercially available from the Consolidated Electrodynamics Corporation and commercially known as a MEG 900 mercury pump. Between the mercury diffusion pump 14 and the mercury diffusion booster pump 12 are a plurality of cooling baffles 16 which prevent fluid from the mercury booster pump 12 from reaching the mercury pump 14. The baffles 16 may be cooled to approximately minus 35 C., by circulating a cooling fiuid such as Freon about the passageway containing bafiles, so that mercury will be condensed in the baffled passageway and drawn back toward the booster pump 12.

Connected to the input of the mercury diifusion pump denses most of the particles of mercury from the mer-.

cury diffusion pump 14 and thus, these condensed mercury particles fall downwardly and back into the mercury diffusion pump 1 The baffles in the region 18 may be cooled by employing Freon as a coolant.

Connected to the input of the Freon baftle 18 is a three-bounce chevron baifie, liquid nitrogen trap 22 in accordance to this invention. The trap 22 will be explained in greater detail in connection with FIG. 2. Connected to the input of the liquid nitrogen trap 22 is a chamber 24 which is to be exhausted to a pressure of approximately 2 l0 mm. of Hg or better.

Referring now specifically to FIG. 2, there is shown.

an enlarged sectional view of theliquid nitrogen trap 22 in accordance with this invention. The trap 22 comprises an elongated hollow cylinder or tube 26. A pair of apertured end plates 26 and 30 are provided each extending across a different end of the cylinder 26. The end plates 28 and 3d are welded to the cylinder 26 at areas 33. The apertured end plate 28, which is positioned at the output end of the cylinder 26 adjacent to the Freon baffle 18 of FIG. 1, includes a hollow cylindrical member or pipe 32 which may be stainless steel and which is welded to the output end plate 28. The pipe 32 is also Welded to a flange 34 (of FIG. 1).

Spaced closely adjacent to (in this case approximately inch) the inner wall of the hollow cylinder 26, is a relatively thin migration baflle and heat radiation shield in the form of a hollow cylinder 40. The migration bathe 4i), which prevents both volume and surface migration of particles, is supported on the input end plate 30 by a vacuum tight weld thereto. Spaced from the output end plate 23 is a flanged end 29 of the migration bafile til through which screws 31 extend. The hollow cylinder 4% is made of a material such as stainless steel which is a poor heat conductor and is also made as thin as possible so that heat conduction axially through the cylinder 44B is maintained at a minimum. Furthermore, the hollow cylinder it? is highly polished for maximum heat reflectivity. v

Connectedto the input end plate 30 is a cooling as sembly comprising an annular tank 43. The tank 48 is formed by a pair of coaxial tubes 49 and 51 having their ends sealed by annular members 53 and 55 which are brazed vacuum tight thereto. The tank 48 may be filled with liquid gas, e.g. liquid nitrogen, by means of a fill pipe or inlet tubing 50, and exhausted through a vent pipe or outlet tubing 52. The pipes 5i) and 52, like cylinder 4d, are of a poor thermal conducting material, such as stainless steel, and have very thin walls. The tank is supported by means of the screws 31 which extend through a high heat conducting compression ring which is in firm contact with the flanged end 29. The tank 48 is made of a high thermal capacity metal, such as copper, so that all portions of the tank 43 will readily be maintained at the temperature of the cooling fluid during operation, even though the tank is only partly filled with cooling fluid. Also, it should be noted that the tank is relatively massive, as compared to its supports and the shield as, so that the entire space enclosed within the inner tube 49 remains substantially at the temperature of the cooling fluid. The inlet and outlet tubes 50 arid 52 may be welded to the annular member 53 andthrough the end plate 30. It should be noted that the liquid nitrogen tank 48 is suspended in a vacuum tight space and is positioned so that the gases being exhausted pass through a plurality of chevron bellies 54, 56 and 58 rather than around the migration baffle 40. The chevron baffles 54, 56 and 58 are also made of a high thermal capacity material, such as copper, and are silver brazed to obtain Patented June 16, 1964 bers or slats, -65, 66 and 67. The rings 65, 66 and 67 are also of 21 t3 good thermal contact with the inner wall of the tank 48 :so-that the bafiles are maintained at the same temperafture as that of the tank and of the cooling medium. The fbaflles 54, 56, and 58 are in the shape of strip-like memeach is supported on. a different metal ring high thermal capacity metal. The baflles in each set are parallel to each other and each baifie extends from opposite wall portions of the inner tube 49. It should be noted that the battles are arranged so that no line of sight travel can occur without the particles bouncing from, for example, baflles 54 to baffles 56 Thus the chevron hafile plates 54, 56, and 58 provide a minimum of three nnolecular bounces, and an average in this embodiment iDf approximately 90 molecular bounces of the gas par- :ticles to be exhausted, as Well as a similar number of lbounces of any particles of mercury which may escape tthe Freon trap 18 in FIG. 1. It is to be especially noted lthat the bafiles 54 are separated from the battles 56, and Ithe baflies 55 are separated from the bafiles 58. Because of this separation between baffles 54, 56, and 58, in accordance with this invention, it is possible to conduct extremely large amounts of gases, as compared to the chevron type traps found in the prior art, where the chevrons were continuous. The reason for this is that the probability of molecular diffusion toward exhaustion through the maze of chevrons, of the type described, is {greatly increased over prior arrangements. For example, ttraps made in accordance with this invention, having a 710 inch opening, have flow rates of approximately 2,000 liters per second. Also, it should be noted that the inlet :and outlet tubes form seals with the annular flange 53, .and with the end plate 30, so that the cooling fluid is confined within the tank 48 without particles of this fluid being exposed to the space within the vacuum chamber of the trap 22.

'The center portion of the end plate 33 is provided with a hollow cylinder or pipe 62, which may be of a material such as stainless steel and which is welded to the end plate 30. One end of the hollow cylinder 62 is adapted to be sealed to the container 24 to be evacuated as shown .in FIG. 1. A specific example of a successfully operated trap 22 is as follows:

Part Length Thick- Mat-e- OD ID ness rial Cylinder 26 18. 25 0. 25 SS 17. 25 17. Shield 40 13. 0. 03 SS 16. 62 16. 59 Outer tube 51. 0.75 Cu 16. 50 16. 75 Inner tube 49 0. 75 Cu 13.00 12. 25 Flange 55.... 0. 50 Cu 16. 50 11.75 Flange 53. 0.37 Cu 16.50 12. 25 Fins 54, 56, 58- 884 0.025 Cu Supports 67. 500 1.000 G11 12. 25 11. 25 Flange 29 062 SS 16. 59 10. 500 End plate 30. .75 SS 17.00 10.500 Inlet pipe 50 030 SS 1.15 1.12 Tube 32.-. any 134 SS 10.500 10.000 Tube 62--. any .109 SS 10.500 10. 000

NOTE .All dimensions represented in inches.

A trap such as that shown in FIG. 2 was made of non-magnetic material, stainless steel, for a particular application and was constructed in such a manner as to permit the trap to be cooled to the temperature of liquid nitrogen or to be baked at a temperature of approximately 450 C. With two of these traps in series in advance of a Freon-cooled bathe, pressures of 2 l0- mm. of Hg or better, have been obtained in the vacuum area 24 for long periods of time. With the use of two traps 22 both of the traps 22 were baked at 450 C., and then the trap nearest the pump was cooled and filled with liquid nitrogen. After several hours of bake out, the outer trap was cooled, the apparatus degassed, and the second trap filled with liquid nitrogen to bring the vessel pressure down to 2X10 mm. of Hg range.

Because the tank 48 and the chevron type handles 54, 56 and 53 are made of a high thermal capacity material, it

has been found that, even though the trap cooling fluid container is only partially full of cooling fluid and the level of the cooling fluid may change, still ultra high vacuums may be achieved. In other words, the entire tank 48 and the chevron bafiles remain substantially at liquid nitrogen temperatures so long as there is liquid nitrogen within the tank 4%. Thus, once the trap has been cooled to liquid nitrogen temperature, and the liquid nitrogen has boiled away, the trap will rise in tempreature less than 10 C. in the following period of 24 hours. This is due to the high thermal capacity of the massive tank.

It should be understood that the trap 48, in accordance with this invention, may include a larger or smaller number of bafiles and a larger or smaller number of chevrons per baflle as long as the chevrons of one bafile are separate from the chevrons of an adjacent hafile, and the chevrons of successive batfies obstruct straight line travel of the molecules of gas.

Thus, this invention provides a novel cooled vacuum trap, one or more of which may be used to provide an improved ultra high vacuum system for yielding extremely low pressures.

What is claimed is:

l. A vacuum trap comprising a hollow tubular member, a cooling fluid tank completely suspended within said tubular member, said tank being made of a high thermal capacity metal, means for supporting said 't'ank withi n said member including a vacuum seal between said tank and said member that forms a vacuum chamber therebe= tween, inlet and outlet means communicating with the in terior of said tank, said tank defining a passageway "therethrough extending in coaxial relation to and communicating with axially spaced regions of said tubular member, and a cooling structure supported in said passageway, said structure including a support engaging the inner wall of said tank, and a plurality of baiiles mounted on said support and disposed in an array to permit air to pass there'- through.

2. A vacuum trap comprising an elongated hollow metallic tubular member, a pair ofend plate members one closing partly only, each end of said tubular member, a cooling fluid tank means completely supported within said tubular member and having an opening therethrough a support, and migration baflle member extending from said cooling fluid tank means to one of said end plate members and forming a vacuum chamber between said tubular member and said tank, and a plurality of bafi le means extending across the opening in said tank means, said battle means including a metal support engaging said fluid tank and a plurality of metallic battles engaging said support and positioned in an air pervious array, whereby said bafiles cool air passing through said array, said tubular member having openings at the ends thereof adapted to be connected to inlet and outlet means for air, said tank means having an opening for admitting a coolant thereto.

3. A vacuum trap as in claim 2 wherein said bafile means are made of copper and said support and migration bafile member is made of stainless steel.

4. A liquid nitrogentrap for cooling evacuated gas passing therethrough, comprising a hollow stainless steel iacket, apertured stainless steel end plates spanning the ends of said jacket, an annular liquid nitrogen tank completely within said jacket, said tank having a support connected to one of said end plates and forming separate, co-axial, cylindrical, oppositely directed, dead-ended; chambers between said jacket and said tank so that, t evacuated gases pass only through the space defin y the inner wall of said tank, a plurality of chevron type battles spanning said space, said baffles and said tank being made of a metal having a higher heat conductivity tha stainless steel, and inlet and outlet tubes f r Said tank sealed through said one of said end plates.

5. A vacuum trap comprisin (a) a chamber which 15 adapted to be evacuated, said chamber having end Walls and e all (b) a duct Connected to a p g each of said end walls and in substantial coaxial relation, (c) an annular tank completely within said chamber having a hollow cylindrical portion in register with said ducts, said tank being spaced from the Walls of said chamber, (d) a plurality of groups of chevron type baflles mounted in said hollow cylindrical portion, and in heat transfer relation to the inner wall of said tank, said groups being disposed in spaced parallel planes, the bafiles in each group being spaced from each other and tilted in a common direction, the bafiles in adjacent groups being tilted in different directions, each group of baflles being spaced from the bafiles in adjacent groups, said annular tank having relatively thick Walls made of a metal of high thermal capacity, (e) inlet and outlet ducts extending through a wall of said tank, for admitting cooling fluid to said tank, and (f) a combined shield and support member surrounding the sides of said tank in spaced relation therewith and fixed to one end wall of said chamber and engaging in hermetically sealed relation the end of said tank remote from said end wall, said member being cylindrical and in coaxial relation with respect to said tank, said member being relatively thin and made of a metal of low thermal capacity, (g) whereby said vacuum trap facilitates a relatively high rate of gas flow therethrough, impedes a surface flow of gases in by-passing relation to said hollow cylindrical portion of said tank, and preserves said tank in thermally insulated relation with respect to the walls of said chamber.

6. A vacuum trap comprising: (a) a chamber having end walls vertically spaced, (b) two ducts sealed through said end walls in substantially coaxial relation for evacuating said chamber, an annular cooling tank completely within said chamber, (d) inlet and outlet means communicating with the interior of said tank, whereby said tank is adapted to receive and to hold cooling fluid, said tank being spaced from the end walls and side Walls of said chamber and having relatively thick walls made of a relatively high thermal capacity metal, whereby a partial filling of said tank with cooling fiuid serves to cool the tank to a substantially uniform temperature, (e) and means between and spaced from said tank and the side walls of said chamber for supporting said tank in said chamber, said supporting means comprising a cylindrical member made of a relatively low thermal capacity metal and having relatively thin walls of one end of said cylindrical member being hermetically fixed to the upper end wall of said chamber, the other end of said member extending downwardly and hermetically fixed to the lower end of said tank, whereby said member tensionally supports said tank and isolates said outer surface of the outer Wall of said tank.

References Cited in the file of this patent UNITED STATES PATENTS 1,433,258 Buckley Oct. 24, 1922 1,535,157 Hughes et al. Apr. 28, 1925 2,386,298 Downing et al. Oct. 9, 1945 2,636,655 McF-ee Apr. 28, 1953 2,703,701 Simpelaar Mar. 8, 1955 2,703,969 Lindsey Mar. 15, 1955 2,865,560 Franceschini Dec. 23, 1958 2,904,665 Gordon Sept. 15, 1959 FOREIGN PATENTS 773,456 Great Britain Apr. 24, 1957 

1. A VACUUM TRAP COMPRISING A HOLLOW TUBULAR MEMBER, A COOLING FLUID TANK COMPLETELY SUSPENDED WITHIN SAID TUBULAR MEMBER, SAID TANK BEING MADE OF A HIGH THERMAL CAPACITY METAL, MEANS FOR SUPPORTING SAID TANK WITHIN SAID MEMBER INCLUDING A VACUUM SEAL BETWEN SAID TANK AND SAID MEMBER THAT FORMS A VACUUM CHAMBER THEREB ETWEEN, INLET AND OUTLET MEANS COMMUNICATING WITH THE INTERIOR OF SAID TANK, SAID TANK DEFINING A PASSAGEWAY THERETHROUGH EXTENDING IN COAXIAL RELATION TO AND COMMUNICATING WITH AXIALLY SPACED REGIONS OF SAID TUBULAR MEMBER, AND A COOLING STRUCTURE SUPPORTED ENGAGING THE INNER WALL OF SAID TANK, AND A PLURALITY OF BAFFLES MOUNTED ON SAID SUPPORT AND ISPOSED INAN ARRAR TO PERMIT AIR TO PASS THERETHROUGH. 